WO 2011/042858 discloses a method of processing a signal including at least a component representative of a periodic phenomenon in a living being includes obtaining at least one first signal having at least a component representative of a periodic phenomenon in a living being. Separate data representative of at least a periodic component of motion of the living being are obtained by obtaining a sequence of images showing the living being, synchronized with the at least one first signal, and carrying out video-based motion analysis of the sequence of images. The data are used at least to suppress a component of the first signal corresponding to the periodic component of motion of the living being.
US 2010/0061596 discloses a method of determining a similarity with a portion of a physiological motion, includes obtaining a first image of an object, obtaining a second image of the object, determining a level of similarity between the first and second images, and correlating the determined level of similarity between the first and second images with a portion of the physiological motion. A computer product having a set of instructions, an execution of which causes a method of determining a similarity with a portion of a physiological motion to be performed, the method includes obtaining a first image of an object, obtaining a second image of the object, determining a level of similarity between the first and second images, and correlating the determined level of similarity between the first and second images with a portion of the physiological motion.
Vital signs of a person, for example the heart rate (HR), blood oxygen saturation (SpO2) or respiratory information (respiratory parameters) such as the respiratory rate (RR), can serve as a powerful predictor of serious medical events. For this reason the respiratory rate and/or the heart rate are often monitored online in intensive care units or in daily spot checks in the general ward of a hospital. Besides the heart rate, the respiratory rate is one of the most important vital signs. Both, the HR and the RR are still difficult to measure without having direct body contact. In present intensive care units, thorax impedance plethysmography or the respiratory inductive plethysmography are still the methods of choice for measuring the RR, wherein typically two breathing bands are used in order to distinguish thorax and abdominal breathing motion of a person. The HR is typically measured by use of electrodes, fixed at the chest of the subject, wherein the electrodes are connected to remote devices through cables. However, these obtrusive methods are uncomfortable and unpleasant for the patient being observed.
It has been shown that one or more video cameras can be used for unobtrusively monitoring the HR, the RR or other vital signs of a subject by use of remote photoplethysmographic (remote PPG) imaging. Remote photoplethysmographic imaging is, for instance, described in Wim Verkruysse, Lars O. Svaasand, and J. Stuart Nelson, “Remote plethysmographic imaging using ambient light”, Optics Express, Vol. 16, No. 26, December 2008. It is based on the principle that temporal variations in blood volume in the skin lead to variations in light absorptions by the skin. Such variations can be registered by a video camera that takes images of a skin area, e.g. the face, while the pixel average over a selected region (typically part of the cheek in this system) is calculated. By looking at periodic variations of this average signal, the heart rate and respiratory rate can be extracted. There are meanwhile a number of further publications and patent applications that describe details of devices and methods for obtaining vital signs of a patient by use of remote PPG.
Thus, the pulsation of arterial blood causes changes in light absorption. Those changes observed with a photodetector (or an array of photodetectors) form a PPG (photo-plethysmography) signal (also called, among other, a pleth wave). Pulsation of the blood is caused by the beating heart, i.e. peaks in the PPG signal correspond to the individual beats of the heart. Therefore, a PPG signal inherently includes a heart rate signal. The normalized amplitude of this signal is different for different wavelengths, and for some wavelengths it is also a function of blood oxygenation or other substances found in blood or tissue.
Compared to conventional contact sensors used for measuring heart rate, respiration and SpO2, which are attached to the subject and who's main source of noise is motion, the main advantages of camera-based vital signs monitoring is the high ease-of-use since there is no need to attach a sensor but the camera just needs to be aimed at the appropriate region of interest, e.g. a skin or chest region of the subject. Another advantage of camera-based vital signs monitoring is the potential for achieving motion robustness since cameras have a significant spatial resolution while contact sensors mostly comprise a single element detector.
One of the key challenges for the remote PPG technology is to be able to provide robust measurement during subject motion, in particular to provide a full motion robust solution.